The effects of the natural and artificial light environment on calciferol (vitamin D) synthesis in the skin will be assessed by seasonal measurements of blood cholecalciferol and ergocalciferol levels in healthy adults in six different cities. These will be correlated with the light environments in which they work, and the sun exposure of the cities in which they live. In one city, correlations will also be made with seasonal measurements of the subjects' calcium metabolism. In addition, the action spectrum for calciferol photoproduction in the skin, and its modification by skin pigmentation, will be established in man and a laboratory animal, using a specific competitive-binding assay for calciferol. Finally, the total endogenous production rate of cholecalciferol, and the efficiency of its hepatic 25-hydroxylation, will be directly measured in laboratory animals and in man, before and after measured irradiation with simulated sunlight. This information can be used to assess critically the biological implications of alterations in the natural or artificial light environment.